Hand Crank Flashlight

This hand crank flashlight charges a supercapacitor to power an LED when you turn the crank. In fact, the hand crank system provides enough power that you can also power the LED directly if the capacitor has run out of charge. This flashlight uses no batteries and converts your own energy into light. It could hypothetically run for decades and is great in an emergency situation.

To learn more about the diodes used in this project, check out the Electronics Class.

For this lesson you will need:

(x1) Stepper motor
(x8) 1N5819 schottky diodes
(x1) 1N4733 5.1V zener diode
(x1) Super-bright white LED
(x1) 15F supercapacitor
(x1) 100 ohm resistor
(x1) 0.1uF capacitor
(x1) 330 ohm resistor
(x1) PCB
(x1) Latching pushbutton switch
(x1) 5mm bore shaft coupling
(x1) 2" lever arm
(x4) 6-32 x 1/2" bolts
(x1) M6 x 20mm bolt
(x1) Reflector
(x1) 1" plastic flashlight lens
(x1) Black binding post
(x1) Rubber washer (optional)
(x1) 5" x 2.5" x 2" project enclosure

The secret sauce to this project is a stepper motor. A stepper motor is a special type of motor that has two power coils. By powering the coils one after another, and then alternating the polarity and powering them again, the motor is able to move. Don't worry too much if this doesn't make too much sense right now. The key word here is "alternating." If you consider that a motor is actually a transducer and can both be powered by electricity, and generate electricity when manually powered, a stepper motor actually has two coils producing alternating current when you turn the motor shaft. Since stepper motors tend to have large magnets and multiple coils, this makes them highly efficient at generating electricity.

All we need to do is crank to the motor shaft. To test this out, add an LED to each pair of motor coils and observe what happens.

Of course, it is not quite that easy. There is one problem to this approach. We are getting alternating current out of the motor, but we are trying to power a DC circuit. This is where diodes start to come in.

By arranging diodes into a bridge rectifier we are able to convert AC electricity to DC. In this arrangement, regardless of where the AC waveform is in its cycle, electricity is always flowing between power and ground in a uniform direction, and you end up with a DC waveform output.

The reason for this is that as the alternating current fluctuates between positive and negative, two diodes are always forward biased and two are reversed biased. Through this clever arrangement, there is always a pathway for electricity to flow only between power and ground.

Since our flashlight has two coils that each generate an AC signal, we need two bridge rectifiers. To get them to work together, we simply wire them in parallel: positive to positive, and ground to ground. We're also going to add a small capacitor to help smooth out the voltage and fill in any dips in voltage created as we cycle between the power created by the two coils.

Once we have a nice clean DC signal coming out of the rectifiers, we will then need to charge a supercapacitor. However, before we do that, we need to make sure that the voltage from the rectifiers will never exceed the voltage of the supercapacitor.

While the voltage coming out of the motor is probably fairly small, to be on the safe side we can use a zener diode to ensure it never exceeds the capacitors 5.6V operating voltage.

By connecting a 5.1V zener diode in series with a 100 ohm resistor in a reverse bias position between power and ground, we can ensure the voltage to the capacitor never exceeds 5.1V.

If the voltage exceeds 5.1V, then the zener effect kicks in limiting the voltage across the diode to 5.1V, and dropping any additional current across the current limiting resistor. For instance, if the motor produces 9V, then 5.1V will flow across the diode and 3.9V will fall across the resistor. Any component connected in parallel to the diode will receive at most 5.1V.

This is not the best means of voltage regulation since it can potentially generate a lot of heat, but since the current we are working with is relatively small, it should be fine.

After the power is generated and regulated, it is then stored in a supercapacitor like we used in the vibrobot project. The capacitor is simply in parallel with the zener diode.

The LED and its current limiting resistor are wired in parallel to the zener diode and supercapacitor.

A switch is then connected in series with the resistor in order to toggle on and off its connection to the supercapacitor power supply.

When the supercapacitor runs out of power, the LED can be powered directly by the hand crank to long as the switch is toggled on. If the switch is toggled off, the capacitor gets charged instead.

To begin with, cut out and attach the drilling template for the stepper motor. It should be taped to the lid of the enclosure on center about 1" from one of its shorter edges.

Drill the outer mounting holes with a 1/8" drill bit and the center hole for the shift with a 3/8" drill bit.

Fasten the motor to the lid using M3 x 12mm bolts.

Widen the hole at the end of the crank arm to be 1/4" wide to receive the bolt for the crank knob.

Insert thread-locking fluid into knob's threading to prevent the bolt from later loosening while you crank the flashlight.

Loosely fasten the knob to the crank arm using an M6 x 20mm bolt. It should be attached loosely enough that the knob will be able to spin in place.

Attach the shaft coupling to the crank arm using four 6-32 bolts.

Fasten the crank arm to the motor shaft using the shaft coupling's set screw.

This completes the motor's crank arm assembly.

Trim the wires coming out of the stepper motor to be about 4" - 6" long.

This will make it easier to work with later and fit in the case.

Drill a hole on center on one of the smallest faces of the enclosure for the flashlight's reflector cone using a 1-1/16" hole saw.

The easiest way to find center of any rectangular surface is to draw an X from corner to corner.

Drill a 1/8" pilot hole in the side of the enclosure near the hole for the reflector, and then widen it to 3/8". This is for mounting the switch.

Now is time to build the circuit on the PCB as pictured in the schematic.

To begin, I wired the two bridge rectifiers in place.

I then connected the zener diode voltage regulator.

After that, I wired in both capacitors.

Lastly, I added the resistor for the LED. The remaining components will be mounted off the board and connected later.

Connect a red wire to the LED's anode and a black wire to its cathode. Insulate the solder joints with shrink tubing to protect them and prevent potential shorts.

Apply contact cement to the reflector and the lip around the edge of the LED.

Wait for both to dry until they are tacky to the touch and then firmly press them together to make a firm bond.

Attach the lens to the reflector by applying contact cement around the front inner lip of the reflector and the outer edge of the lens.

Again, wait for both to dry until they are tacky to the touch and press them together firmly.

Pass the switch up through the hole in the enclosure and fasten it in place with its mounting hardware.

Apply contact cement on the enclosure around the outer edge of the reflector mounting hole, and the edge of the reflector.

Press the two firmly together once the contact cement is dry enough.

Solder the motor's wires to the appropriate rectifiers on the circuit board. If you have forgotten where this is, reference the schematic.

Also, make sure you get the wire pairs correct. In my case red and blue were attached to one coil, and black and green were attached to the other coil. If you mix it up and take one wire from each coil - let's say, black and red - then not much will happen.

Solder the LED in series with the resistor on the circuit, and the switch in series with the power supply.

Solder the switch and LED together to complete the circuit. Now, when the switch is pressed the LED is either connected or disconnected from the supercapacitor, enabling you to turn it on and off.

Cut two small adhesive-back velcro tabs and mount the circuit board neatly on the inside of the enclosure in a spot that will be out of the way of the servo motor.

Put the lid back on the case, and fasten it shut with the enclosure's mounting screws.

Attach the rubber lens cover gasket with contact cement over the edge of the lens to seal it up.

Give your flashlight a couple minutes of solid cranking, and enjoy no longer being in the dark.